Adapted from the popular stage play of the same name, Dear Ruth features Mona Freeman as teenaged Miriam Watkins, who can't keep her nose out of other people's affairs. Fired up by patriotism, Miriam inaugurates a warm pen-pal relationship with an overseas air force officer (William Holden), hinting at a future marriage. When the airman arrives in town, he insists upon seeing Miriam's older sister Ruth (Joan Caulfield). It seems that Miriam, in an effort to appear older, signed her letters with her sister's name, and even enclosed her sister's picture.